Two stroke internal combustion engine

ABSTRACT

A two stroke internal combustion engine having a compressed air inlet port and an exhaust port the flow through which is controlled by suitable valves. Fuel is injected through a fuel injector. The timing of the opening of the valve is such that, as the piston approaches top dead centre and with the air inlet valve closed, the exhaust valve is closed such that some exhaust gas is trapped and compressed in the combustion chamber thereby increasing the temperature within the combustion chamber and hence facilitating ignition. The invention also contemplates initiating combustion before the air inlet valve is closed.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/GB01/01471 which has an Internationalfiling date of Mar. 30, 2001, which designated the United States ofAmerica.

The present invention relates to a two stroke internal combustionengine. More particularly, the invention relates to the processes offuel ignition and combustion in such an engine.

It has long been recognised that the compression stroke of aconventional reciprocating internal combustion engine requiressignificant amounts of energy, which to a certain extent can be regardedas a parasitic drain on the performance of the engine. One of thereasons for this is that the air induced into a conventional engine isheated by contact with the hot cylinder head, liner and piston. The workof compression is significantly increased by any process which increasesthe temperature.

The compression work can be reduced by compressing the air in a separatecylinder which can be much cooler than the cylinders in which thecombustion occurs. A further benefit of external compression of the airis that it is possible to save fuel by pre-heating the compressed airwith hot engine exhaust gases. Such an arrangement is disclosed in U.S.Pat. No. 4,300,486 which describes a two or four stroke engine in whichcompressed air is supplied to the combustion cylinders from a storagetank. The air is compressed efficiently using power from a sourceexternal to the engine.

A development of this idea is disclosed in U.S. Pat. No. 4,476,821 whichhas a reciprocating compressor mechanically coupled to the crank shaftof the engine to provide a constant supply of compressed air to theinlet of each combustion cylinder.

U.S. Pat. No. 4,040,400 discloses a similar arrangement in which thecompression is done as a two stage process with intercooling between thetwo stages to improve efficiency.

The above concepts were taken a stage further in WO 94/12785 whichdiscloses a recuperated two stroke engine including an air compressioncylinder with an intense water spray injection, followed by a downstreamwater separator. The water spray not only cools the air throughout thecompression in order to minimise the work of compression, but alsoallows a higher compression ratio than can be achieved with conventionalreciprocating air compressors. This reduces the number of compressionstages required. Indeed, only one stage would normally be required.

In all reciprocating internal combustion engines in which there isexternal air compression and air preheating, it is necessary to mix thehot, compressed air with fuel and to ignite the mixture in thecombustion cylinder(s) of the engine. Depending on the type of fuel,this may not be easy to accomplish in a stable and reliable way. Inparticular, if the fuel is natural gas, which has a high ignitiontemperature and a relatively long ignition delay, then it is verydifficult to pre-heat the air sufficiently to achieve reliableauto-ignition and combustion. It is of course possible to achieveignition of natural gas by the use of a pilot fuel, which will ignite atpractical air pre-heat temperatures. Indeed conventional dual fuelengines and gas diesels use this method, but there are disadvantages interms of the costs of the extra equipment and storage of the pilot fuel.Also significant is the effect of the pilot fuel in increasing theemission of nitrogen oxides.

Another common method of igniting natural gas mixtures is to use sparkignition, but this is normally used only where the natural gas isalready mixed with the air prior to entry into the cylinder. Sparkignition of premixed air and fuel works well in small engines having anear-stoichiometric air-fuel ratio, but is harder to achieve in modernlarge engines employing lean burn combustion to achieve reduced NOxemissions. In this case, the energy of the spark needs to besupplemented by the use of ignition in a small pre-chamber. The mixturein the pre-chamber is not lean, but at near stoichiometric proportions,and can therefore be ignited by the spark to produce a jet of burninggas, which has sufficient energy to ignite the main mixture. Apart fromthe extra complexity of the pre-chamber, the use of spark ignition hasthe disadvantage that the environment of the pre-chamber is very hostileto the spark plugs and they have to be replaced at frequent intervals.

The present invention therefore aims to provide a way of igniting fuel,which is not premixed, in a two stroke internal combustion engine withexternal air compression which provides reliable combustion, a simpleconstruction with low maintenance, and the ability to use a wide rangeof combustible fuels including those which are hard to ignite such asnatural gas.

According to the present invention a two stroke internal combustionengine comprises at least one cylinder, the or each cylinder having apiston reciprocably movable within the cylinder and defining acombustion chamber, the or each combustion chamber having a compressedair inlet port with an air inlet valve for controlling flow into thecombustion chamber, a fuel injector through which fuel is injected intothe combustion chamber, and an exhaust port with an exhaust valve forcontrolling flow out of the combustion chamber; and control means forcontrolling the air inlet and exhaust valves in relation to the motionof the piston such that the exhaust valve is closed substantially beforethe piston reaches top dead centre and substantially before the airinlet valve is opened, such that some exhaust gas is trapped andcompressed in the combustion chamber.

The exhaust gas is by its nature already hot (normally 500-800° C.).Once the exhaust valve is closed and with the air inlet valve closed,the piston continues to move to top dead centre and compresses theexhaust gas within the combustion chamber. For a given pressure ratio,the absolute temperature after compression is proportional to theabsolute temperature at the start of compression. By selecting anappropriate pressure ratio, the compressed exhaust gas can be raised toa high enough temperature that it can be made to ignite even fuels oflow ignition quality such as natural gas.

The invention can be carried out without requiring a spark ignition, apre-chamber or a pilot fuel. Therefore the construction of the enginecan be kept simple and the need for maintenance is reduced.

The opening and closing motion of inlet and exhaust valves in aninternal combustion engine needs to be carefully controlled. This isbecause if a valve is seated too quickly, significant damage or rapidwear may occur if the contact is too sudden. Further, valves willgenerally be operated by a cam and again care must be taken with regardto the contact between the cam and valves. For this reason, the liftprofile of an exhaust or inlet valve shows a gentle ramping up at thestart of lift to allow the valve actuator to begin to move the valvewithout damage. Similarly, a gentle ramp down is provided as the valveis closed so that the valve is seated gently. The ramps may occupy asignificant proportion of the piston cycle (typically 20° of crankangle). However, during such time very little flow will occur past thevalve element. Therefore, to all intents and purposes, the valve elementis closed during this time, even though it is not physically fullyseated. Therefore, for the avoidance of doubt, for the sake of thisdocument, the valve is not considered to be open until it has achieved5% of its maximum lift. Similarly, the valve is deemed to be closed onceit has returned to the same position. This definition applies throughoutthis specification to the claims and description alike. Thus, allreferences to the valve opening time refer to the time at which thevalve opens beyond the 5% lift position. Equally, references to valveclosing times refer to the point at which the valve reaches the 5%position on closing.

The relationship between the closing of the exhaust valve, the openingof the air inlet valve, and the piston position is dependent uponseveral characteristics of the engine and the fuel. These include theresidual volume within the cylinder when the piston is at top deadcentre, the inlet air temperature and pressure, the type of fuel beingused, the temperature of the exhaust gas, the ignition quality of thefuel and the desired temperature of the exhaust gas after compression.Nevertheless, the control means is preferably configured such that thegap between the closure of the exhaust valve and the opening of the airinlet valve is at least 10°, more preferably at least 15°, and mostpreferably at least 20° of crank angle of a nominal crank shaft drivingthe piston. The exhaust valve is preferably closed at least 10°, morepreferably at least 15°, and most preferably at least 20° of crank angleof a nominal crank shaft driving the piston before top dead centre ofthe piston. The control means may be configured to open the air inletvalve substantially at top dead centre. However opening of the air inletvalve preferably begins just before (e.g. 3° of crank angle before) topdead centre. This valve opening position is beneficial because theinitial inrush of air completes the recompression of the exhaust gas,which has already been partly done by the piston. If the piston isdesigned to do all of the recompression, then the pressure within thecylinder could exceed that of the air supplied in the port and causereverse flow through the valve. It is better to allow some margin toavoid this. In fact, simulations of the process show that it is alsoslightly more efficient to allow the air to complete the final stages ofrecompression. Recompression by the incoming air has a very similareffect to recompression by the piston, with one major difference, whichis that the incoming air mixes with the compressed gas so that thetemperature rise is less than would be achieved without the mixing. Iftoo much recompression is done by the incoming air instead of by thepiston, then the temperature rise would not be sufficient to ignitecertain fuels.

A further advantage of allowing the incoming air to complete therecompression is that the air inlet valve achieves a greater opening attop dead centre.

The control means could be configured to change the opening of theexhaust valve while the engine is running in response to changing engineconditions, but it is simpler to find an optimum setting for the exhaustvalve opening which is not adjusted during operation.

The control means is preferably configured to inject fuel into thecombustion chamber at substantially the same time as the compressed airis introduced into the combustion chamber. The control means preferablycontrols the timing of the fuel injection and the profile of the fuelflow rate. Further, it has been found to be beneficial for the controlmeans to inject a small amount of fuel into the combustion chamber afterthe exhaust valve has closed and before top dead centre. The smallquantity of fuel mixes with the high temperature compressed exhaust gasand initiates combustion. This process of ignition is different from theconventional use of a pilot fuel to ignite fuels of low ignitionquality, not only because the initiating mechanism is different but alsobecause in the present situation there is no difference between the fuelwhich initiates combustion and the main fuel. Also in the presentsituation the same fuel injector injects both the fuel that initiatescombustion and the main fuel. The hot exhaust gas contains some oxygenwhich was not consumed in the previous cycle. Some additional oxygen maybe available from hot air, which enters the cylinder through the airinlet valve prior to top dead centre, although this is not essential tothe process. The availability of oxygen combined with the hightemperature produced by the recompression of the exhaust gases causesignition of the pre-injected fuel, with a finite but acceptable delay.

The compressed air supplied to the combustion chamber may be providedfrom a reservoir of compressed air. However, it is preferable for thecompressed air to be generated on demand by a compressor, which ispreferably a reciprocating compressor driven by the or each piston,preferably via a crank shaft.

For greatest efficiency, the compressor is an isothermal compressor, anda means for heating the compressed air is provided upstream of thecompressed air inlet port. The means for heating may be an externalheater, but, more efficiently, is a heat exchanger fed with exhaust gasfrom the combustion chamber which gives up its heat to the compressedair flowing from the compressor to the combustion chamber.

The isothermal compressor is preferably of the type comprising acylinder in which a further piston is reciprocably movable and defines acompression chamber, an air inlet port with an inlet valve forcontrolling flow into the compression chamber, a compressed air outletport with an outlet valve for controlling flow out of the compressionchamber, means for spraying liquid into the compression chamber duringthe compression stroke of the further piston, and a separator provideddownstream of the compressed air outlet port to separate the liquid fromthe compressed air. The means for spraying liquid is preferablyconfigured such that, during compression, heat is transferred to theliquid droplets as sensible heat substantially without evaporation ofthe droplets, since this permits a lower temperature of air duringcompression.

The compression of the exhaust gas takes place in the combustion chamberwhich is defined between the walls and head of the cylinder and the topsurface of the piston. In its simplest form, with a flat-topped piston,the final clearance volume is essentially a very short cylindricalspace. Such a space has a high surface to volume ratio and a significantfraction of heat produced by the exhaust gas compression can be lost.Also, it can be difficult to mix the fuel with the trapped exhaust gasin such a flat, wide clearance volume. Preferably, therefore, a recessis provided in the cylinder head or top piston surface into which recessthe fuel is directed when the piston is near the top of its stroke. Mostconveniently, this recess is provided by a piston bowl in the uppersurface of the piston.

Piston bowls are used in conventional diesel engines to encourage mixingof fuel and air to improve the burning of the fuel. In larger dieselengines, the bowl increases the flow length for vaporisation of atomisedliquid sprays. Diesel engines, which employ air compression within thecombustion cylinder, must leave sufficient space above the piston forall this air. Thus the trapped volume between piston top and cylinderhead at top dead centre is necessarily larger than in engines withexternal air compression. The shape of the piston bowl in diesel engineshas relatively little effect on the ignition process, but it isimportant for the progress of combustion following ignition.

The primary purpose of the piston bowl when used with the presentinvention, which applies only to an engine with external aircompression, is to facilitate ignition, rather than burning, of fuelssuch as natural gas without the use of a spark plug, pre-chamber or aseparate pilot fuel. This is achieved partly by reduction of heat lossesper unit mass of exhaust gas caused by the lower surface to volume ratioand partly by the enhancement of mixing as exhaust gas is rapidlysqueezed out of the surrounding clearances into the bowl during thecompression stage. In practical terms, the trapped volume at top deadcentre is much smaller than in most diesel engines, since it isdesirable to re-compress the minimum amount of residual exhaust gasconsistent with achieving ignition. Indeed, the volume of the pistonbowl is determined by this requirement. The trapped volume within thecylinder at top dead centre including the piston bowl and all clearancespreferably amount to less than 3% and more preferably less than 2% ofthe total cylinder volume at bottom dead centre. In comparison, in alarge conventional diesel engine, the trapped volume within the cylinderat top dead centre would be 5% or more of the cylinder volume at bottomdead centre. When a piston bowl is provided, every attempt should bemade to reduce the amount of trapped volume which is not accounted forby the piston bowl, since additional compression work has to be done inthe gas in this volume to no useful purpose. In practice, however, it islikely that the volume of trapped gas outside the bowl (at top deadcentre) may be of the same order as that trapped inside the bowl.

In order to avoid contact between the air inlet valve and the piston andat the same time to minimise the dead space at top dead centre, it ispreferable for the air inlet valve to be arranged to open withoutprotruding into the combustion chamber. If the air inlet valve is apoppet valve, this implies that the valve should open in the directionaway from the combustion chamber. On the other hand, as the exhaustvalve is shut while the piston is close to top dead centre, it can be aconventional inwardly opening valve.

According to a second aspect of the present invention there is provideda method of operating a two stroke internal combustion engine comprisingat least one cylinder, the or each cylinder having a piston reciprocablymovable within the cylinder and defining a combustion chamber, the oreach combustion chamber having a compressed air inlet port with an airinlet valve for controlling flow into the combustion chamber, a fuelinjector, and an exhaust port with an exhaust valve for controlling theflow out of the combustion chamber; the method comprising repeating thesteps of:

-   opening the exhaust valve; moving the piston into the combustion    chamber to force exhaust gas from the combustion chamber; closing    the exhaust valve before the piston reaches top dead centre and    trapping some exhaust gas in the combustion chamber; compressing the    exhaust gas by further movement of the piston; initiating the    opening of the air inlet valve; and introducing fuel into the    combustion chamber once the exhaust gas has been compressed to the    extent that the temperature in the combustion chamber is sufficient    to ignite and combust the fuel expanding the hot combustion gases so    as to perform work on the piston.

This method can be used with any combustible fuel, but is best suited tofuels of low ignition quality such as natural gas.

In order to start the engine it is first of all necessary to get the oreach piston moving within its cylinder. It is then necessary to providesome alternative way of igniting the fuel within the combustion chamberas before the first ignition no exhaust gas is available forcompression.

If an external source of power is conveniently available, for example ina power generating installation where power from the grid electricalsupply is available, this can be used to start the pistons moving. Onthe other hand, if no such external power supply is available, theengine must have its own means of moving the pistons. Preferably, theengine further comprises a reservoir of compressed air and a heater, amethod of starting the engine comprising the steps of heating air fromthe compressed air reservoir, feeding the hot compressed air, into theor each combustion chamber, and expanding the hot compressed air in theor each combustion chamber in order to move the piston down thecylinder.

Once the pistons are moving, it is then necessary to ignite the fuel.The hot compressed air which expands in the combustion chamber asdescribed above to do work against the piston will cool substantially bythe time the piston is at bottom dead centre, since no heat will beadded by combustion of fuel. The subsequent compression of the fractionof this air which remains in the combustion chamber after the exhaustvalve closes will heat the air up again to something like its originaltemperature, but this may not be hot enough to ignite the fuel. In orderto start the engine, particularly with a fuel of low ignition qualitysuch as natural gas, it is necessary to ensure that the low pressure airin the cylinder prior to re-compression is at a similar temperature tothe supplied high pressure air.

The start-up method therefore preferably comprises bleeding a flow ofhot, compressed air through a restricted aperture into the combustionchamber at a time when cylinder pressure is low. The restricted apertureprovides a throttling effect, which, as throttling is an isenthalpicprocess, will cause the temperature after throttling to remainessentially unchanged. Even if the piston is on its downstroke, suchthat the cylinder volume is expanding, the bleed flow does not dosignificant work against the piston since the air pressure in thecylinder remains low during the whole process. To achieve the objectiveof throttling the air into the cylinder while the pressure is still lowallows a choice of timing of the hot air injection during starting. Theair can be admitted before the exhaust valve opens, during the periodthat it is open and/or immediately after the exhaust valve has closed.The choice of timing of the hot air injection during starting may beaffected by whether hot compressed air is also being expanded within thecylinder. As mentioned previously, the expansion of hot compressed airmay be used as a method of rotating the engine during start-up. Howevereven if other means are available to rotate the engine, hot compressedair could be admitted to the cylinder and expanded simply as a means ofwarming up the pipework and the engine. If hot compressed air isexpanded in the cylinder for any reason, then there would be no purposein throttling bleed air into the cylinder during the expansion, since itwould mix with the cool expanded air. From the point of view ofconserving the amount of hot air used for starting, admission of the hotbleed air during and immediately after closing the exhaust valve wouldbe the most attractive option, whichever method is used to turn theengine.

The hot compressed air used for ignition during start-up may be suppliedfrom any source, but if hot compressed air is also used to turn theengine during start-up, then preferably both would be supplied from thesame compressed air reservoir and heater arrangement.

The restricted aperture may be provided by a throttle valve in thecylinder head, or the gap between the air inlet valve and the valve seatif the air inlet valve is arranged to open by a fraction of its normallift.

During the start-up operation, the exhaust valve is opened and closed asnormal so that the piston compresses the already hot bleed air as itapproaches top dead centre thus further increasing its temperature.Several strokes of the piston would be performed, recompressing bleedair at every stroke, before the first fuel is injected in order toestablish the process and warm up the system.

Once the engine is firing, the restricted aperture is closed therebyshutting off the bleed flow. If the throttle valve is used as therestricted aperture, this is simply a matter of closing the valve. Ifthe gap between the air inlet valve and valve seat is used, the gap isclosed simply by returning the air inlet valve to its normal operationby fully seating the air inlet valve until it is required to open toadmit the main charge of air.

The restricted aperture may be closed immediately the engine beginsfiring. However, if the restricted aperture is closed gradually, thebleed air is gradually diminished before being stopped, and this canprovide for a smoother transition between start-up and normal operation,thereby improving the reliability of the start-up process.

In normal operation, the combustion of the fuel may be initiated in eachsuccessive cycle after the air inlet valve is closed to try to achieveconstant volume combustion similar to that in a conventionalreciprocating engine. However, it has been found preferable for thecontrol means to be arranged to control the air inlet valve in relationto the combustion in the combustion chamber such that the air inletvalve is not closed before combustion is initiated.

This goes against conventional teaching, as it results in some loss ofpressure in the combustion chamber. However, it has been found that thisdisadvantage is more than offset by the fact that this allows time forthe combustion process. Also, the conventional approach of havingconstant volume combustion results in a steep pressure rise within thecombustion chamber which increases the temperature of the combustiongases and hence the NOx emissions. Initiating combustion with thepressure rise reduced results in a consequent reduction in NOxemissions.

Preferably, the control means is configured such that under normaloperating conditions the gap between the initiation of combustion andthe inlet valve being closed is at least 15°, preferably at least 25°,and more preferably at least 30°, of crankangle of a nominal shaftdriving the piston.

This particular feature forms an independent aspect of the presentinvention, which can broadly be defined as a two stroke internalcombustion engine comprising at least one cylinder, the or each cylinderhaving a piston reciprocably movable within the cylinder and defining acombustion chamber, the or each combustion chamber having a compressedair inlet port with an air inlet valve for controlling flow into thecombustion chamber, a fuel injector through which fuel is injected intothe combustion chamber, and an exhaust port with an exhaust valve forcontrolling flow out of the combustion chamber; and control means forcontrolling the air inlet valve in relation to the combustion in thecombustion chamber, such that the air inlet valve is not closed beforecombustion is initiated.

In order to enhance the advantage provided by initiating combustionbefore closing the air inlet valve, the control means is configured suchthat under normal operating conditions the gap between the initiation ofcombustion and the air inlet valve being closed is at least 15°,preferably at least 25°, and more preferably at least 30° of crankangleof a nominal crankshaft driving the piston.

Preferably, the control means is configured to inject a small amount offuel into the combustion chamber before top dead centre.

Alternatively, this aspect of the invention can be defined as a methodof operating a two stroke internal combustion engine comprising at leastone cylinder, the or each cylinder having a piston reciprocably movablewithin the cylinder and defining a combustion chamber, the or eachcombustion chamber having a compressed air inlet port with an air inletvalve for controlling flow into the combustion chamber, a fuel injectorand an exhaust port with an exhaust valve for controlling the flow outof the combustion chamber; the method comprising repeating the steps of:

-   opening the exhaust valve; moving the piston into the combustion    chamber to force exhaust gas from the combustion chamber; closing    the exhaust valve; injecting fuel, initiating the opening of the air    inlet valve and initiating combustion; and subsequently closing the    air inlet valve, such that the air inlet valve is not closed before    combustion is initiated.

In a conventional large diesel engine, the fuel is normally injectedinto the combustion chamber through a central multi-hole injector, withholes arranged symmetrically around the circumference of the injector.In such engines, the fuel is normally directed at a downward angle intoa bowl in the piston. However, computer studies of combustion behaviourhave shown that this type of nozzle arrangement is not appropriate forthe type of engine described in which the air is compressed and heatedexternally and then introduced into the cylinder essentiallysimultaneously with the injection of the fuel. The problem is that aftertop dead centre the incoming air from the air inlet valve rapidly fillsmost of the volume created by the downwardly moving piston and displacesthe mixture of air, combustion gases and fuel to the far side of thechamber. Mixing of the incoming air with the pre-existing gases and thefuel is poor and combustion is relatively slow.

In order to overcome this problem, it is preferable that the or eachinlet valve is associated with an air inlet port, the or each air inletport being associated with an imaginary column defined as the envelopeformed by the translation of the area of the inlet port in the directionin which the piston reciprocates; the fuel injector being arranged todirect at least 50%, preferably at least 70% and more preferably 100% ofthe fuel towards the column or columns.

This forms an independent aspect of the present invention which can beused together with or independently of the previous aspects of theinvention and is broadly defined as a two stroke internal combustionengine comprising at least one cylinder, the or each cylinder having: apiston reciprocably movable within the cylinder and defining acombustion chamber, the or each combustion chamber having at least onecompressed air inlet port with an associated air inlet valve forcontrolling flow into the combustion chamber, the or each air inlet portassociated with an imaginary column defined as the envelope formed bythe translation of the area of the inlet port in the direction in whichthe piston reciprocates; at least one exhaust port with an exhaust valvefor controlling flow out of the combustion chamber; and a fuel injectorthrough which fuel is injected into the combustion chamber, the fuelinjector being arranged to direct at least 50% and preferably at least70% and more preferably 100%, of the fuel towards the column or columnsbut not towards the or each air inlet valve.

Alternatively, this aspect of the invention can be defined as a methodof operating a two stroke internal combustion engine comprising at leastone cylinder, the or each cylinder having a piston reciprocably movablewithin the cylinder and defining a combustion chamber, the or eachcombustion chamber having: at least one compressed air inlet port withan air inlet valve controlling flow into the combustion chamber, eachport being associated with an imaginary column defined as the envelopeformed by the translation of the area of the inlet port in the directionin which the piston reciprocates; at least one exhaust port with anexhaust valve for controlling the flow out of the combustion chamber;and a fuel injector for injecting fuel into the combustion chamber; themethod comprising the step of injecting at least 50%, preferably atleast 70% and more preferably 100% of the fuel towards the column orcolumns but not towards the or each air inlet valve.

With this arrangement some or all of the fuel is directed into a regiondirectly below the or each air inlet port as defined by the column orcolumns. This region is essentially that having the air jet or jets,which are produced with the air inlet valve or valves open. The fuelenters the turbulent air stream, is dispersed within it and then carriedwith the air to all parts of the combustion chamber. This relies heavilyon the movement of the incoming air to distribute the fuel. The rapidmixing of fuel and air, which is achieved by this approach, is alsobeneficial from the point of view of NOx production, since the premixingof air and fuel before combustion dilutes the fuel with excess air andgas and moderates the rise in temperature. This is particularlyeffective when combustion takes place at near constant pressure.

In order to enhance the effect of using the turbulent air stream todisperse the fuel, the fuel is preferably injected at a shallow anglewith respect to the cylinder head. Preferably, the fuel injector injectsthe fuel at an angle of less than 10° and preferably less than 5° withrespect to the cylinder head.

In most cases where each combustion chamber has more than one inletport, these will be arranged on one side of the cylinder head with theexhaust ports being arranged on the other side. This is important forthe geometry of the porting and the pipework outside the cylinder. Inthis case, at least 80% of the fuel, and preferably at least 90% wouldbe injected towards the side of the cylinder which contains the airinlet ports.

An engine constructed in accordance with the present invention will nowbe described with reference to the accompanying drawings, in which:

FIG. 1 is a schematic representation of the various elements of theengine;

FIG. 2 is a set of graphs illustrating the cylinder pressure, fuelinjection, inlet valve position and exhaust valve position as a functionof crank angle; and

FIG. 3 is a schematic plan view of a cylinder head showing the locationof various components.

The engine components shown in FIG. 1 are a schematic representation ofcomponents that can be found in FIG. 4 of WO 94/12785.

The engine comprises a combustor in the form of, for example, threecylinders each with a reciprocating piston and with internal combustionof fuel in a two-stroke cycle. The three cylinders are representedschematically in FIG. 1 by a single cylinder 1 and reciprocating piston2. The upper surface of the piston 2 is provided with a piston bowl 2Alocated adjacent to fuel injector 5. The pistons 2 of the threecombustion cylinders are connected to a common crankshaft (not shown)which is of conventional construction. Each cylinder has two compressedair inlet valves 3, two exhaust outlet valves 4 and a fuel injector 5(note that only one of each type of valve is shown in FIG. 1).

The combustion cylinders 1 are provided with a supply of hot compressedair. This is generated in isothermal compressor 6 which is areciprocating compressor in which a piston reciprocates within acylinder to cause compression. The compression piston may be connectedto the same crankshaft as the combustion cylinders 1. During thecompression process, water is injected into the isothermal compressor 6in order to maintain the compression process as close as possible toisothermal. A suitable arrangement of nozzles to achieve isothermalcompression is disclosed in WO 98/16741.

The cold compressed air and water are then fed to a separator 7 wherethe bulk of the water is separated from the compressed air. Thecompressed air which is now substantially free of water is fed along theline 8 via a recuperator 9 where it receives heat from exhaust gas fromthe combustion cylinders 1 before entering one of the combustioncylinders 1. All of the elements of the engine described thus far, withthe exception of the piston bowl 2A, are present in FIG. 4 of WO94/12785.

The unique feature of the present invention is in the control of thetiming of the valves of the combustor in order to achieve anadvantageous ignition process. The control of the timing of the valvesis done, for example, with a camshaft in which the cam lobes havesuitable profiles to achieve the necessary timing. If the valves areelectromagnetically, hydraulically, or pneumatically actuated the sameeffect can be achieved with a suitable control circuit.

The timing of the cylinder pressure, fuelling, inlet valve opening andexhaust valve opening is shown in FIG. 2, and a complete two-strokecycle of a single combustion cylinder will now be described based onthis drawing. For several combustion cylinders, each will follow thesame process with a suitable phase delay.

The exhaust valve begins to open perhaps 20 crankangle degrees beforebottom dead centre so that there is a substantial opening by the timebottom dead centre is reached. There is a rapid depressurisation of thecylinder or blowdown at this time. Once the cylinder pressure hasequalised with the pressure in the recuperator, the depressurisationceases. Throughout the majority of the up-stoke of the piston, thecylinder pressure is similar to that in the low pressure side of therecuperator, while the piston 2 forces the exhaust gas out of thecylinder 1 past exhaust valve 4.

The exhaust valve begins to close during the piston upstroke, reachingeffective closure (i.e. less than 5% lift) about 25° before top deadcentre as shown in FIG. 2D, trapping the remaining exhaust gas in thecombustion chamber. Even before this point is reached, there is somepressure rise and after this point there is a substantial furtherincrease in pressure to about 60% of the pressure of the air supplied tothe air inlet port. This pressure rise is shown in FIG. 2A. The pressurerise is accompanied by a substantial temperature increase. The finalpressure rise up to the pressure of the inlet air occurs very rapidlyjust before top dead centre when the inlet valve starts to open. Thiseffect is also shown in FIG. 2A. This final pressure rise also causes afurther increase in temperature of the trapped exhaust gas, whichprovides further assistance to the ignition process. On the other hand,the incoming air, which causes this final recompression is cooler, sothat the mixed mean temperature is not increased very much. At this timethe fuel injector opens briefly in order to inject a small amount offuel, as shown in FIG. 2(B), which combines with unburnt oxygen in thehot exhaust gas to ignite in the high temperature environment producedby the compression of the exhaust gas.

The inlet valve 3 begins to open shortly before top dead centre as shownby the solid line in FIG. 2(C) to allow hot compressed air into thecombustion cylinder 1. At the same time the fuel injector injects themain charge of fuel into the bowl 2A as shown in FIG. 2(B). The fuelinjection process may terminate either slightly before the air inletvalve is closed as shown in FIG. 2B and FIG. 2C or it may terminateslightly after air inlet valve closure. This depends on the engineloading, the pressure ratio of the combustor expansion, the speed of theengine and the time delay between fuel injection and combustion. Themixture of the hot compressed air and the main charge of fuel is ignitedby the flame resulting from the self-ignition of the fuel injectedearlier. The hot combustion gases expand and perform work as the piston2 is driven downwards on the power stroke. As shown in FIG. 2A, there isa reduction in pressure in the combustion cylinder 1 as the expansionproceeds. Just before bottom dead centre, the exhaust valve begins toopen in preparation for the next stroke as shown in FIG. 2(D).

The mechanism for starting the engine on fuels of low ignition qualitywill now be described. If the engine can be driven by externalmechanical or electrical power, this is used to drive the isothermalcompressor in order to pressurise the pipework upstream of the combustorair inlet. If no external drive is available, then the pipework may bepressurised using a reservoir of stored compressed air 10, shown in FIG.1. The compressed air reservoir is preferably situated upstream of therecuperator, since the pipework at this point does not get too hot whenthe engine is running normally. This avoids the need to design valveswhich are capable of withstanding very high temperatures.

An air heater is needed at start-up since no heat is available from therecuperator. A convenient method of heating the air supplied to thecombustor is to heat the pipework, which runs between the recuperatorand the combustor. The heat input required for the purpose of startingthe engine is very much less than that which is supplied by therecuperator during normal operation because the air mass flow is so muchlower. This is because the rotational speed of the engine is low andbecause the pressure of air supplied to the compressor is nearatmospheric pressure and is not boosted by the operation of aturbocharger during start up. As shown in FIG. 1, a heater 11 isconfigured around this pipework. The heater may be any one of variousforms of electrical heater or it may be a gas or oil burner.

If the engine can be driven by external power during start up then thepistons are already moving up and down in their respective cylinders. Ifthe engine is not driven by external power, the compressed air from thereservoir, which is also heated in the connecting pipework, is expandedin the cylinders in order to provide the power to move the pistons.

Once the pistons are moving it is necessary to ignite the fuel withouthaving any exhaust gas available for recompression to generate therequired high temperature.

This is done by providing a bleed flow of compressed air for a period oftime during the closing of the exhaust valves. This may be done througha restricted orifice leading into the combustion chamber cylinder 1, butin the case illustrated in FIG. 1 this can be done by opening the inletvalve 3 briefly during the final stages of exhaust valve closure (whenthe lift is less than 5% of maximum) as indicated by the broken line inFIG. 2C. Since the engine speed during start-up is much lower thanduring normal operation it is found that the valve opening required toadmit the required air is either very small or exists only briefly intime. If the valve opens too much or is open for too long a duration,then too much air may be admitted. This air with added fuel may then besubsequently forced back into the inlet port as the piston continues torise, which is most undesirable. Another disadvantage is that thecompressor, which is likely also to be running at slow speed, may not beable to sustain the required air flow. The exhaust valve is not yetfinally closed at this point, and because the engine speed is low andthe pressure of the air supplied to the inlet valve is high, it ispossible to consume a significant quantity of air if the valve durationis too long. Thus FIG. 2C illustrates a brief admission of air occurringat about 20° before top dead centre.

If the combustor cylinder has more than one air inlet valve, then it maybe convenient to use one valve to provide the brief air injection duringstart-up and use the remaining valve or valves to provide the main airflow after top dad centre. This avoids the need for one valve to openand close twice in the starting cycle.

An alternative to the above procedure for igniting the engine directlyusing fuel of low ignition quality would be to provide the engine withthe facility to start-up on fuel of high ignition quality such as lightdiesel fuel. The engine would run for sufficient time to allow therecuperator to heat up and then it would be switched over to the fuel oflow ignition quality.

The injection of the fuel into the combustion chamber will now bedescribed with reference to FIGS. 1 and 3. FIG. 3 shows the cylinderhead 12 on one side of which are a pair of compressed air inlet ports 13each of which is associated with a compressed air inlet valve 3 and onthe other side of which are a pair of exhaust gas outlet ports 14 eachof which is associated with an exhaust outlet valve. The fuel injector 5has a pair of outlet orifices 15 which are arranged to direct fuel inthe direction of arrows 16 towards the compressed air inlet ports 13. Aswill be apparent from FIG. 1, the fuel is directed at a slight downwardincline with respect to the cylinder head. However, the fuel canalternatively be directed parallel to the cylinder head. The fuelinjection is timed so that fuel is injected as air is entering thecombustion chamber through the air inlet ports 13 as shown in FIG. 2.

Although FIGS. 1 and 3 indicate that all the fuel is injected into theair jets entering the cylinder, with one injection orifice 15 per airinlet port 13, this is not necessarily always the case. In some cases itmay be preferable to have a larger number of nozzle holes, for exampleto distribute the fuel within the air jet, or to avoid the possibilityof the fuel penetrating straight through the air jet. The optimumarrangement will also depend on whether the injected fuel is a gas or aliquid.

As shown in FIGS. 1 and 3, the fuel injector 5 normally protrudes intothe piston bowl 2A, which must therefore be positioned around theinjector. The injection of fuel directly towards the air inlet ports 3in the asymmetric manner described above may be thought to cause most ofthe fuel to escape from the bowl 2A thus negating or reducing the effectof the piston bowl in achieving ignition of fuels which are hard toignite. However this is not the case, since ignition is achieved at atime when the piston 2 is very close to the top of the cylinder 1 andmost of the air, gas and fuel, which is present in the cylinder issqueezed out from the surrounding narrow clearances into the piston bowl2A. Thus at this early stage of the combustion process, the piston bowlwill still perform the function that it is designed for.

1. A two stroke internal combustion engine comprising at least onecylinder, the or each cylinder having a piston reciprocably movablewithin the cylinder and defining a combustion chamber, the or eachcombustion chamber having a compressed air inlet port with an air inletvalve for controlling flow into the combustion chamber, a fuel injectorthrough which fuel is injected into the combustion chamber, and anexhaust port with an exhaust valve for controlling flow out of thecombustion chamber; and control means for controlling the air inlet andexhaust valves in relation to the motion of the piston such that theexhaust valve is closed substantially before the piston reaches top deadcentre and substantially before the air inlet valve is opened, such thatsome exhaust gas is trapped and compressed in the combustion chamber. 2.An engine according to claim 1, wherein the control means is configuredsuch that the gap between the closure of the exhaust valve and theopening of the air inlet valve is at least 10° of crank angle of anominal crank shaft driving the piston.
 3. An engine according to claim1 or claim 2, wherein the control means is configured such that the gapbetween closing the exhaust valve and top dead centre is at least 10° ofcrank angle of a nominal crank shaft driving the piston.
 4. An engineaccording to claim 1 or claim 2 wherein the control means is configuredto open the inlet valve just before top dead centre.
 5. An engineaccording to claim 1, wherein the control means is configured to injectfuel into the combustion chamber at substantially the same time as thecompressed air is introduced into the combustion chamber.
 6. An engineaccording to claim 1, wherein the control means is configured to injecta small amount of fuel into the combustion chamber after the exhaustvalve has closed and before top dead centre.
 7. An engine according toclaim 6, wherein the small amount of fuel is of the same type as thatused for the main injection and is introduced through the fuel injector.8. An engine according to claim 1, further comprising a compressor forthe supply of compressed air to the combustion chamber.
 9. An engineaccording to claim 8, wherein the compressor is a reciprocatingcompressor driven by the or each piston.
 10. An engine according toclaim 8 or claim 9, wherein the compressor is an isothermal compressorand the engine further comprises a means for heating the compressed airupstream of the compressed air inlet port.
 11. An engine according toclaim 10, wherein the means for heating is a heat exchanger fed withexhaust gas from the combustion chamber which gives up its heat to thecompressed air flowing from the compressor to the combustion chamber.12. An engine according to claim 10, wherein the isothermal compressorcomprises a cylinder in which a further piston is reciprocably movableand defines a compression chamber, an air inlet port with an inlet valvefor controlling flow into the compression chamber, a compressed airoutlet port with an outlet valve for controlling flow out of thecompression chamber, means for spraying liquid into the compressionchamber during the compression stroke of the further piston, and aseparator provided downstream of the compressed air outlet port toseparate the liquid from the compressed air.
 13. An engine according toclaim 12, wherein the means for spraying liquid is configured such that,during compression, heat is transferred to the liquid droplets assensible heat substantially without evaporation of the droplets.
 14. Anengine according to claim 1, wherein, in the combustion chamber, arecess is provided in the cylinder head or top piston surface into whichrecess the fuel is directed when the piston is near the top of itsstroke.
 15. An engine according to claim 14, wherein the recess is apiston bowl in the upper surface of the piston.
 16. An engine accordingto claim 15, wherein the trapped volume in the cylinder at top deadcentre including the piston bowl and all clearances amount to less than3% of the total cylinder volume at bottom dead centre.
 17. An engineaccording to claim 1, wherein the air inlet valve does not protrude intothe cylinder when the valve is open.
 18. An engine according to claim15, wherein the trapped volume in the cylinder at top dead centreincluding the piston bowl and all clearances amount to less than 2%. 19.An engine according to claim 1, wherein a restricted aperture isprovided to allow a throttled flow of hot compressed air into thecombustion chamber and a valve is provided to control the flow ofcompressed air through the restricted aperture on engine start-up. 20.An engine according to claim 19, wherein the restricted aperture is aseparate opening in the cylinder.
 21. An engine according to claim 19,wherein the restricted aperture is the gap between the air inlet valveand the valve seat when the air inlet valve is partially open.
 22. Amethod of starting up an engine according to claim 19, the start-upmethod comprising bleeding a flow of hot, compressed air through therestricted aperture into the combustion chamber at a time when thecylinder pressure is low.
 23. A method of starting up an engineaccording to claim 22, wherein the compressed air is heated by a heaterwhich heats a pipe supplying the compressed air to the combuster.
 24. Amethod according to claim 23, wherein the hot compressed air is bledinto the combustion chamber during the final stages of closing of theexhaust valve.
 25. A method according to claim 24 further comprisingclosing the restricted aperture gradually once the fuel is ignited. 26.A method according to claim 23, further comprising shutting off therestricted aperture once the fuel is ignited.
 27. An engine according toclaim 1, wherein the control means are arranged to control the air inletvalve in relation to the combustion in the combustion chamber such thatthe air inlet valve is not closed before combustion is initiated.
 28. Anengine according to claim 27, wherein the control means is configuredsuch that under normal operating conditions the gap between theinitiation of combustion and the inlet valve being closed is at least15° of crankangle of a nominal shaft driving the piston.
 29. An engineaccording to claim 27, wherein the control means is configured such thatunder normal operating conditions the gap between the initiation ofcombustion and the inlet valve being closed is at least 25° ofcrankangle of a nominal shaft driving the piston.
 30. An engineaccording to claim 27, wherein the control means is configured such thatunder normal operating conditions the gap between the initiation ofcombustion and the inlet valve being closed is at 30° of crank angle ofa nominal shaft driving the piston.
 31. An engine according to claim 1,wherein the or each inlet valve is associated with an air inlet port,the or each air inlet port being associated with a column defined as theenvelope formed by the translation of the area of the inlet port in thedirection in which the piston reciprocates; the fuel injector beingarranged to direct at least 50% of the fuel towards the column orcolumns.
 32. An engine according to claim 1, wherein the engine is not aspark ignited engine.
 33. A method of starting up an engine according toclaim 1 and further comprising a reservoir of compressed air and aheater, the method of starting up the engine comprising the steps ofheating air from the compressed air reservoir, feeding the hotcompressed air into the or each combustion chamber, and expanding thehot compressed air in the or each combustion chamber in order to movethe piston out of the combustion chamber.
 34. An engine according toclaim 1, wherein the control means is configured such that the gapbetween the closure of the exhaust valve and the opening of the airinlet valve is at least 15° of crank angle of a nominal crank shaftdriving the piston.
 35. An engine according to claim 1, wherein thecontrol means is configured such that the gap between the closure of theexhaust valve and the opening of the air inlet valve is at least 20° ofcrank angle of a nominal crank shaft driving the piston.
 36. An engineaccording to claim 1 or claim 2, wherein the control means is configuredsuch that the gap between closing the exhaust valve and top dead centreis at least 15° of crank angle of a nominal crank shaft driving thepiston.
 37. An engine according to claim 1 or claim 2, wherein thecontrol means is configured such that the gap between closing theexhaust valve and top dead centre is at least 20° of crank angle of anominal crank shaft driving the piston.
 38. An engine according to claim1, wherein the or each inlet valve is associated with an air inlet port,the or each air inlet port being associated with a column defined as theenvelope formed by the translation of the area of the inlet port in thedirection in which the piston reciprocates; the fuel injector beingarranged to direct at least 70% of the fuel towards the column orcolumns.
 39. An engine according to claim 1, wherein the or each inletvalve is associated with an air inlet port, the or each air inlet portbeing associated with a column defined as the envelope formed by thetranslation of the area of the inlet port in the direction in which thepiston reciprocates; the fuel injector being arranged to direct 100% ofthe fuel towards the column or columns.
 40. A method of operating a twostroke internal combustion engine comprising at least one cylinder, theor each cylinder having a piston reciprocably movable within thecylinder and defining a combustion chamber, the or each combustionchamber having a compressed air inlet port with an air inlet valve forcontrolling flow into the combustion chamber, a fuel injector, and anexhaust port with an exhaust valve for controlling the flow out of thecombustion chamber; the method comprising repeating the steps of:opening the exhaust valve; moving the piston into the combustion chamberto force exhaust gas from the combustion chamber; closing the exhaustvalve before the piston reaches top dead centre and trapping someexhaust gas in the combustion chamber; compressing the exhaust gas byfurther movement of the piston; initiating the opening of the air inletvalve; and introducing fuel into the combustion chamber once the exhaustgas has been compressed to the extent that the temperature in thecombustion chamber is sufficient to ignite and combust the fuelexpanding the hot combustion gases so as to perform work on the piston.41. A method according to claims 40 wherein the fuel is natural gas. 42.A method according to claim 40 or claim 2 wherein the exhaust valve isclosed at least 10° of crank angle of a nominal crank shaft driving thepiston before the air inlet valve is opened.
 43. A method according toclaim 40, wherein the exhaust valve is closed at least 10° of crankangle of a nominal crank shaft driving the piston before top dead centreof the piston.
 44. A method according to claim 43, wherein the air inletis opened just before top dead centre.
 45. A method according to claim43, further comprising injecting fuel into the combustion chamber atsubstantially the same time as the compressed air is introduced into thecombustion chamber.
 46. A method according to claim 45, furthercomprising injecting a small amount of fuel into the combustion chamberafter the exhaust valve has closed and before top dead centre.
 47. Amethod according to claim 46, wherein the small amount of fuel is of thesame type as that used for the main injection and is injected throughthe fuel injector.
 48. A method according to claim 43, wherein the stepof introducing the compressed air is done without the air inlet valveprotruding into the combustion chamber.
 49. A method according to claim43, further comprising the step of initiating combustion before closingthe air inlet valve.
 50. A method according to claim 49, wherein undernormal operating conditions combustion is initiated at least 15° beforethe inlet valve is closed.
 51. A method according to claim 49, whereinunder normal operating conditions combustion is initiated at least 25°before the inlet valve is closed.
 52. A method according to claim 49,wherein under normal operating conditions combustion is initiated atleast 30° before the inlet valve is closed.
 53. A method according toclaim 40 or claim 41, wherein the exhaust valve is closed at least 15°of crank angle of a nominal crank shaft driving the piston before theair inlet valve is opened.
 54. A method according to claim 40, or claim41, wherein the exhaust valve is closed at least 20° of crank angle of anominal crank shaft driving the piston before the air inlet valve isopened.
 55. A method according to claim 40, wherein the exhaust valve isclosed at least 15° of crank angle of a nominal crank shaft driving thepiston before top dead centre of the piston.
 56. A method according toclaim 40, wherein the exhaust valve is closed at least 20° of crankangle of a nominal crank shaft driving the piston before top dead centreof the piston.
 57. A two stroke internal combustion engine comprising atleast one cylinder, the or each cylinder having a piston reciprocablymovable within the cylinder and defining a combustion chamber, the oreach combustion chamber having a compressed air inlet port with an airinlet valve for controlling flow into the combustion chamber, a fuelinjector through which fuel is injected into the combustion chamber, andan exhaust port with an exhaust valve for controlling flow out of thecombustion chamber; and control means for controlling the air inletvalve in relation to the combustion in the combustion chamber, such thatthe air inlet valve is not closed before combustion is initiated.
 58. Anengine according to claim 57, wherein the control means is configuredsuch that under normal operating conditions the gap between theinitiation of combustion and the air inlet valve reaching its closedposition is at least 15° of crankangle of a nominal crankshaft drivingthe piston.
 59. An engine according to claim 58, wherein the or eachinlet valve is associated with an air inlet port, the or each air inletport being associated with a column defined as the envelope formed bythe translation of the area of the inlet port, in the direction in whichthe piston reciprocates; the fuel injector being arranged to direct atleast 50% of the fuel towards the column or columns.
 60. An engineaccording to claim 57 or claim 43, wherein the control means isconfigured to inject a small amount of fuel into the combustion chamberbefore top dead centre.
 61. An engine according to claim 58, wherein theor each inlet valve is associated with an air inlet port, the or eachair inlet port being associated with a column defined as the envelopeformed by the translation of the area of the inlet port in the directionin which the piston reciprocates; the fuel injector being arranged todirect at least 70% of the fuel towards the column or columns.
 62. Anengine according to claim 58, wherein the or each inlet valve isassociated with an air inlet port, the or each air inlet port beingassociated with a column defined as the envelope formed by thetranslation of the area of the inlet port in the direction in which thepiston reciprocates; the fuel injector being arranged to direct at least100% of the fuel towards the column or columns.
 63. An engine accordingto claim 57, wherein the control means is configured such that undernormal operating conditions the gap between the initiation of combustionand the air inlet valve reaching its closed position is at least 25° ofcrankangle of a nominal crankshaft driving the piston.
 64. An engineaccording to claim 57, wherein the control means is configured such thatunder normal operating conditions the gap between the initiation ofcombustion and the air inlet valve reaching its closed position is atleast 30° of crankangle of a nominal crankshaft driving the piston. 65.A method of operating a two stroke internal combustion engine comprisingat least one cylinder, the or each cylinder having a piston reciprocablymovable within the cylinder and defining a combustion chamber, the oreach combustion chamber having a compressed air inlet port with an airinlet valve controlling flow into the combustion chamber, a fuelinjector and an exhaust port with an exhaust valve for controlling theflow out of the combustion chamber; the method comprising repeating thesteps of: opening the exhaust valve; moving the piston into thecombustion chamber to force exhaust gas from the combustion chamber;closing the exhaust valve; injecting fuel, initiating the opening of theair inlet valve and initiating combustion; and subsequently closing theair inlet valve, such that the air inlet valve is not closed beforecombustion is initiated.
 66. A method according to claim 65, whereincombustion is initiated under normal operating conditions at least 15°before the air inlet valve is closed.
 67. A method according to claim 46or claim 66, wherein a small amount of fuel is injected prior to topdead centre.
 68. A method according to claim 65, wherein combustion isinitiated under normal operating conditions at least 25° before the airinlet valve is closed.
 69. A method according to claim 65, whereincombustion is initiated under normal operating conditions at least 30°before the air inlet valve is closed.
 70. A two stroke internalcombustion engine comprising at least one cylinder, the or each cylinderhaving a piston reciprocably movable within the cylinder and defining acombustion chamber, the or each combustion chamber having: at least onecompressed air inlet port with an associated air inlet valve forcontrolling flow into the combustion chamber, the or each air inlet portassociated with a column defined as the envelope formed by thetranslation of the area of the inlet port in the direction in which thepiston reciprocates; at least one exhaust port with an exhaust valve forcontrolling flow out of the combustion chamber; and a fuel injectorthrough which fuel is injected into the combustion chamber, the fuelinjector being arranged to direct a total of at least 50% of the fueldirectly towards individual ones of the column or columns but nottowards the or each air inlet valve.
 71. A engine according to claim 70,wherein the fuel injector is arranged to direct at least 70% of the fueltowards the individual ones of the column or columns.
 72. An engineaccording to claim 71, wherein the or each combustion chamber has two ormore compressed air inlet ports, all of which are arranged on the sameside of the cylinder, and wherein the fuel injector is arranged todirect at least 80%, of the fuel towards the side of the cylinder whichcontains the air inlet ports.
 73. An engine according to claim 70 or 50,wherein the fuel injector is arranged to inject the fuel at an angle ofless than 10° with respect to the cylinder head.
 74. An engine accordingto claim 71, wherein the or each combustion chamber has two or morecompressed air inlet ports, all of which are arranged on the same sideof the cylinder, and wherein the fuel injector is arranged to direct atleast 90% of the fuel towards the side of the cylinder which containsthe air inlet ports.
 75. A engine according to claim 70, wherein thefuel injector is arranged to direct at least 70% of the fuel towards theindividual ones of the column or columns.
 76. A engine according toclaim 70, wherein the fuel injector is arranged to direct 100% of thefuel towards the individual ones of the column or columns.
 77. An engineaccording to claim 70 or 71, wherein the fuel injector is arranged toinject the fuel at an angle of less than 50° with respect to thecylinder head.
 78. A method of operating a two stroke internalcombustion engine comprising at least one cylinder, the or each cylinderhaving a piston reciprocably movable within the cylinder and defining acombustion chamber, the or each combustion chamber having: at least onecompressed air inlet port with an air inlet valve controlling flow intothe combustion chamber, each port being associated with a column definedas the envelope formed by the translation of the area of the inlet portin the direction in which the piston reciprocates; at least one exhaustport with an exhaust valve for controlling the flow out of thecombustion chamber; and a fuel injector for injecting fuel into thecombustion chamber; the method comprising the step of injecting at atotal of at least 50% of the fuel directly towards individual ones ofthe column or columns but not towards the or each air inlet valve.
 79. Amethod according to claim 78, comprising the step of directing at least70% of the fuel towards the individual ones of the column or columns.